Electrochromic system

ABSTRACT

A method for the manufacture of an electrochromic system using a nanoporus-nanocrystalline film comprising a semiconducting metallic oxide having a redox chromophore adsorbed thereto.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/952,867,filed Sep. 12, 2001, now U.S. Pat. No. 6,605,239 which is a continuationof application Ser. No. 09/367,024, filed Oct. 26, 1999 now U.S. Pat.No. 6,301,038, issued Oct. 9, 2001, which was based on a Internationalapplication no. PCT/IE98/00008, with an international filing date Feb.6, 1998, having a priority based on Irish patent application no.S970082, filed Feb. 6, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to improvements in or relating toelectrochromic systems.

As the energy performance of buildings and automobiles becomes anincreasingly important design feature, strategies for optimisingperformance in this respect are receiving considerable attention.

An important aspect of the energy performance of the above relates tothe incident radiation transmitted by the window area of a building.Such concerns are further complicated by the need to ensure occupantcomfort. It is in this context that the electrochromic (EC) windowtechnology has assumed increasing importance, the amount of incidentradiation transmitted by such windows being electronically controllable.Effective implementation of EC window technology in buildings isexpected to provide the following benefits:

1. Reduce adverse cooling effects. Reduce cooling energy. Down-size airconditioning plant. Reduce peak electricity demand.

2. Increase beneficial effects of daylight. Reduce lighting energy.Reduce peak electricity demand.

3. Increase occupant comfort. Increase thermal comfort. Increase visualcomfort.

Even greater benefits would be expected to accrue in an automobile,where the ratio of glazed surface to enclosed volume is significantlylarger than in a typical building. Specifically, effectiveimplementation of EC window technology in automobiles is expected toprovide the following benefits in addition to those in the builtenvironment:

1. Increased motoring safety. Reduced glare. Mirror control. Head-updisplay.

EC technology is not limited to the applications described above. Othersinclude privacy glass, angle-independent high-contrast large-areadisplays, glare-guards in electronic devices, electronic scratch pads.

Existing EC devices, including those commercially available, arenon-optimal for the large glazing areas encountered in building andautomotive applications and are based on technologies which are processand energy intensive. Therefore new EC technologies, resulting inimproved device specification and which may be manufactured more easilyat a lower cost, will be commercially important. It is noted in thiscontext, that the current market for EC window technologies in buildingsand automobiles is estimated world-wide at over $2 billion.

For an overview of these and related topics see the review Large-AreaChromogenics:Materials and Devices for Transmittance Control (Eds.Lampert and Granqvist), SPIE Institutes for Advanced OpticalTechnologies Series Vol. 4. Existing EC devices are found in one of thetwo categories outlined below. Firstly, there are those devices based onion insertion reactions at metal oxide electrodes. To ensure the desiredchange in transmittance the required number of ions must be intercalatedin the bulk electrode to compensate the accumulated charge. However, useof optically flat metal oxide layers requires bulk intercalation of ionsas the surface area in contact with electrolyte is not significantlylarger than the geometric area. As a consequence the switching times ofsuch a device are typically of the order of tens of seconds.

Secondly, there are those devices based on a transparent conductingsubstrate coated with a polymer to which is bound a redox chromophore.On applying a sufficiently negative potential there is a transmittancechange due to formation of the reduced form of the redox chromophore. Toensure the desired change in transmittance a sufficiently thick polymerlayer is required, the latter implying the absence of an intimatecontact between the transparent conducting substrate and a significantfraction of the redox chromophores in the polymer film.

As a consequence the switching times of such a device are, as above,typically of the order of tens of seconds.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved ECsystem.

According to the invention there is provided ananoporous-nanocrystalline film comprising a semiconducting metallicoxide having a redox chromophore adsorbed thereto.

A “nanocrystalline film” is constituted from fused nanometer-scalecrystallites. In a “nanoporous-nanocrystalline” film the morphology ofthe fused nanocrystallites is such that it is porous on thenanometer-scale. Such films, which may hereinafter be referred to asnanostructured films, typically possess a surface roughness of about1000 assuming a thickness of about 10 μm.

The nanostructured films used in the present invention colour onapplication of a potential sufficiently negative to accumulate electronsin the available trap and conduction band states. As a consequence ofthe high surface roughness of these films, ions are readilyadsorbed/intercalated at the oxide surface permitting efficient chargecompensation and rapid switching, i.e. the need for bulk intercalationis eliminated. However, despite the rapid switching times in such films,the associated change in transmittance is not sufficient for acommercial device. To overcome this limitation a redox chromophore isadsorbed at the surface of the transparent nanostructured film which,when reduced, increases the extinction coefficient of an accumulatedtrapped or conduction band electron by more than an order of magnitude.Further, due to the nanoporous structure and associated surfaceroughness of the nanocrystalline films used, the redox chromophore iseffectively stacked as in a polymer film, while at the same timemaintaining the intimate contact with the metal oxide substratenecessary to ensure rapid switching times.

The redox chromophore may be any suitable redox chromophore andpreferably comprises a compound of the general formula I

wherein X is a charge balancing ion such as a ion such as a halide; R1is any one of the following:

and

R₂ is any one of the following:

 wherein R₁ is as defined above, R₃ is any of the formulae (a) to (f)given above under R₂, m is an integer of from 1 to 6, preferably 1 or 2and n is an integer of from 1 to 10, conveniently 1 to 5.

A particularly preferred redox chromophore is a compound of formula II,viz. bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride

The semiconducting metallic oxide may be an oxide of any suitable metal,such as, for example, titanium, zirconium, hafnium, chromium,molybdenum, tungsten, vanadium, niobium, tantalum, silver, zinc,strontium, iron (Fe²+ or Fe³+) or nickel or a perovskite thereof. TiO₂,WO₃, MoO₃, ZnO and SnO₂ are particularly preferred.

The invention also provides an electrochromic system comprising: a firstelectrode disposed on a transparent or translucent substrate; a secondelectrode; an electrolyte; an electron donor; and an electrochromiclayer comprising a nanoporous-nanocrystalline film according to theinvention intermediate the first and second electrodes.

The substrate is suitably formed from a glass or a plastics material.Glass coated with a conducting layer of fluorine doped tin oxide orindium tin oxide is conveniently used in the EC system of the presentinvention.

The electrolyte is preferably in liquid form and preferably comprises atleast one electrochemically inert salt optionally in molten form insolution in a solvent. Examples of suitable salts includehexafluorophosphate, bis-trifluoromethanesulfonate,bis-trifluoromethylsulfonylamidure, tetraalkylammonium,dialkyl-1,3-imidazolium and lithium perchlorate. Examples of suitablemolten salts include trifluoromethanesulfonate, 1-ethyl, 3-methylimidazolium bis-trifluoromethylsulfonylamidure and 1-propyldimethylimidazolium bis-trifluoromethylsulfonylamidure. Lithium perchlorate isparticularly preferred.

The solvent may be any suitable solvent and is preferably selected fromacetonitrile, butyronitrile, glutaronitrile, dimethylsulfoxide,dimethylformamide, dimethylacetamide, N-methyloxazolidinone,dimethyl-tetrahydropyrimidinone, γ-butyrolactone and mixtures thereof.

The electron donor is preferably a metallocene or a derivative thereof.The electron donor is preferably soluble in the electrolyte solvent.Ferrocene is particularly preferred.

DESCRIPTION OF THE DRAWINGS

The EC system prepared as described in the Example is illustrated inFIGS. 1-3 and the legends are presented in FIGS. 4-9, as follows:

FIG. 1 is a schematic view of the prepared film disposed on a substrate;

FIG. 2 is a schematic view of the prepared electrochromic systemincluding the film shown in FIG. 1;

FIG. 3 is an exploded view of the electrochromic system of FIG. 2;

FIG. 4 shows: (a) Absorption spectrum of the EC system 10 in lowtransmittance state. (b) Test result of modified EC system 10 in (a)after 1 and 10000 test cycles.

FIG. 5 shows: (a) Change in transmittance at 600 nm of the EC system 10in FIG. 4 during 10000 test cycles. (b) Change in colouring and clearingtimes of the EC system 10 in (a) during 10000 test cycles.

FIG. 6 shows: (a) Change in transmittance at 600 nm of modified ECsystem 10 after 10000 test cycles as a function of the firing time ofnanostructured film. (b) Test results of modified EC system 10 in (a)after 10000 test cycles.

FIG. 7 shows: (a) Change in transmittance at 600 nm of modified ECsystem 10 after 10000 test cycles as a function of the dying time ofnanostructured film. (b) Test results of modified EC system 10 in (a)after 10000 test cycles.

FIG. 8 shows: (a) Change in transmittance at 600 nm of modified ECsystem 10 containing 0.20 mol.dm⁻³ LiClO₄ during 10000 test cycles. (b)Change in colouring and clearing times of modified EC system 10 in (a)during 10000 test cycles.

FIG. 9 shows: (a) Change in transmittance at 600 nm of modified ECsystem 10 containing 0.05, 0.10 and 0.20 mol.dm⁻³ ferrocene during 10000test cycles. (b) Change in colouring and clearing times of modified ECsystems 10 in (a) during 10000 test cycles.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated in the following Example.

(a) A 2.5 cm×2.5 cm transparent nanostructured film, consisting of a 4μm thick layer of fused TiO₂ nanocrystallites, was deposited on a 3.3cm×3.3 cm fluorine doped tin oxide on glass substrate (Glastron, TradeMark). A colloidal TiO₂ dispersion was prepared by hydrolysis oftitanium tetraisopropoxide. The average diameter of the initially formedcrystallites (7 nm) was increased by autoclaving at 200° C. for 12 hoursto 12 nm. Concentrating the autoclaved dispersion to 160 g/l and addingCarbowax (Trade Mark) 20000 (40% wt. equiv. of TiO₂) yielded a whiteviscous sol. (Carbowax 20000 is an ethylene glycol polymer whose averagemolecular weight is 20000.) A 4 μm thick layer of the above sol wasdeposited using a screen printing technique on the conducting glasssubstrate. The resulting gel-film was dried in air for 1 h, sintered at450° C. for 12 h and stored in a darkened vacuum desiccator prior touse.

(b) A redox chromophore, bis-(2-phosphonoethyl)-4,4′-bipyridiniumdichloride was prepared by adding 4,4′-bipyridine (4.4 g) anddiethyl-2-ethylbromo-phosphonate (15.0 g) to water (75 ml). The reactionmixture was refluxed for 72 h and allowed to cool. Following addition ofconc. hydrochloric acid (75 ml) the reaction mixture was refluxed for afurther 24 h. To recover the product, the reaction mixture wasconcentrated to 50 ml, isopropyl alcohol (200 ml) added drop-wise,stirred on ice for one hour and filtered. The white crystalline productwas washed with cold isopropyl alcohol and air dried to give purebis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride (12.72 g, 84.24%yield). Calculated for bis-(2-phosphonoethyl)-4,4′-bipyridiniumdichloride (C₁₄H₂₀N₂Cl₂O₆P₂):C, 37.77; H, 4.53; N, 6.29. Found: C,35.09; H, 4.49; N, 6.09. ¹H NMR (water-d₂): δ2.31-2.43 (m, 4H);δ4.68-4.80 (m, 4H); δ8.33 (d, unresolved metacoupling, 4H); δ8.94 (d,unresolved metacoupling, 4H).

(c) TiO₂ films, prepared as described above, were modified by adsorptionof the redox chromophore prepared above from an aqueous solution (0.02mol.dm⁻³) over 24 h, washed with distilled deionised water, dried in airand stored in a darkened vacuum desiccator for 48 h prior to use.

(d) Using a screen printing technique, a 0.25 cm border of a proprietaryepoxy resin (Araldite, Trade Mark) was deposited on a second 3.3×3.3 cmfluorine doped tin oxide conducting glass, leaving a small opening inone corner. This piece of conducting glass was placed on top of themodified TiO₂ film prepared as described above and left to set for 24 h.

(e) To complete construction of the EC system, the above sandwichstructure was back-filled using an argon pressure with an electrolytesolution consisting of LiClO₄ (0.05 mol.dm⁻³) and ferrocene (0.05mol.dm⁻³) in γ-butyrolactone (m.p. −45° C., b.p. 204° C.). Thecomponents of the electrolyte solution were carefully purified andrigorously dried prior to use. The opening was subsequently closed usingAraldite (Trade Mark).

With reference to the drawings and in particular to FIG. 1, there isshown a first glass substrate 11 having a conductive layer 13 offluorine doped tin oxide coated thereon. The exposed surface of thelayer 13 is coated with a transparent nanostructured film 14 of TiO₂having a redox chromophore 15 adsorbed thereon. The redox chromophore 15is bis-(2-phosphonoethyl)-4,4-bipyridinium dichloride prepared asdescribed in the Example.

FIGS. 2 and 3 illustrate an EC system 10 according to the inventioncomprising the first glass substrate 11 with the layer 13 and themodified TiO₂ film 14 shown in FIG. 1 and a second glass substrate 22having a conductive layer 23 of fluorine doped tin oxide coated thereon.The second glass substrate 22 has a 0.25 cm border 24 of epoxy resindeposited thereon with a small gap 25, which is sealed after addition ofthe electrolyte/electron donor solution 16 described above.

It will be observed that construction of the EC system 10 according tothe invention is simple and utilises low-cost and non-toxic materials.These are particularly attractive features in the context of thelarge-scale manufacture of the EC system 10.

It should also be noted that due to surface roughness, of the order of500 for a 4 μm film, no spacer is required in an EC system of theinvention.

In prior art electrochromic systems, a dielectric spacer must beincluded to isolate the electrodes electrically from each other. In thepresent invention, no such spacer is required because the solid particlenature of the nanocrystalline film provides for sufficient electricalisolation between the electrodes. In a commercial version of the ECsystem according to the invention, the absence of a spacer will have apositive impact on the manufacturing costs of the system.

A number of EC systems prepared as described in the above Example weretested by applying 10000 cycles (15 s at −1.00V and 15 s at +1.0V atroom temp.). A typical set of test results is shown in FIGS. 4a and 4b.

Specifically, shown in FIG. 4a are the absorption spectra in the lowtransmittance (LT) state, after 1 and 10000 cycles. It will be observedthat this spectrum, as expected, corresponds to that of the radicalcation of the viologen moiety of the redox chromophore. It will also benoted that, in practice, this corresponds to an intense blue colorationof the EC system and that the extent of this coloration is notdiminished after 10000 cycles.

Concerning the change in transmittance, this is conventionallyrepresented as in FIG. 4b. Specifically, it will be observed that thetransmittance decreases from about 70% (point a, 10000 cycles) to about8% (point b, 10000 cycles). Significantly, the transmittance hasdecreased to less than 20% of its initial value in less than 1 s.Similar behaviour is observed upon switching the EC system to the hightransmittance (HT) state.

As stated, there is no significant degradation in performance over 10000cycles. This is quantitatively demonstrated in FIGS. 5a and 5 b.Specifically, shown in FIG. 5a is the change in transmittance measuredafter 1, 10, 100, 1000 and 10000 cycles. It should be noted that theslight improvement in performance observed is a reproducible feature.Shown in FIG. 5b, are plots of the switching times (as defined above)for the same EC system. These are consistently between 0.9 s and 1.1 s.

Nanostructured TiO₂ films were deposited on the following conductingglass substrates: Indium tin oxide glass and fluorine doped tin oxideglass. No significant difference in the performance in the resulting ECsystem was detected. The time for which a film is fired is important forthe following reason: If a film is fired for 1 h its porosity, andconsequently its surface roughness, will be optimal. However, under thesame conditions, film conductivity will be less than optimal due toincomplete sintering of the constituent nanocrystallites. Conversely, ifa film is fired for 168 h, its connectivity, and consequently itsconductivity, will be optimal.

However, under the same conditions, film porosity will be less thanoptimal due to collapse of the film's nanostructure as will be observedin FIGS. 6a and 6 b. Shown in FIG. 6a are the transmittance changesafter 10000 cycles on switching an EC system in which the constituentnanoporous-nanocrystalline film has been fired for the indicated time.The best performance is obtained for systems containing films that havebeen fired for 12 h. However, as can be seen from FIG. 6b, while thereis improved colouring on going from 6 to 12 h firing time there is nocorresponding decrease in the colouring or clearing time. Film thicknesswas 4 μm or less.

The film firing temperature should be above about 400° C. to remove theadded Carbowax, the addition of which is essential to ensure a porousfilm, and less than 500° C. to prevent conversion of anatase to rutile,the latter being a significantly poorer conductor. For these reasons thefiring temperature was fixed at approximately 450° C.

The substituent groups of the redox chromophore are irreversiblychemisorbed at Ti⁴+ sites at the surface of the TiO₂ nanocrystallitesthat constitute the nanoporous-nanocrystalline film. These substituentgroups, referred to as linker groups, serve, therefore, to irreversiblyattach the redox chromophore to the surface of thenanoporous-nanocrystalline film. The density of these states (about5×10¹³.cm⁻²), and the surface roughness, (about 1000 for a 4 μm film)provide the upper limit for the number of molecular amplifiers which maybe adsorbed per unit geometric area. It should also be noted that,unlike previous linkers, there is no discoloration of the modified filmdue to the existence of a charge transfer interaction between theoccupied molecular orbitals of the linker and the available conductionband stated of the semiconductor substrate. With regard to the redoxchromophore, the viologen moiety is stable with a large associatedchange in extinction for a one electron reduction. Further, the redoxchromophore may be readily modified to change its electrochemical andoptical properties by use of the various substituents associated with Rin the general formula. Each variation possesses different formalpotentials and different colours upon being switched. Furthermore, theredox chromophore may be readily prepared with high yield in a pure formand, perhaps most importantly, adsorbed onto the TiO₂ substrate from anaqueous solution.

One parameter which was studied in respect of the redox chromophore wasthe extent of modifier adsorption in a given period. As would beexpected, the redox chromophore is adsorbed to an increasing extent frommore concentrated solutions in a shorter time. In practice, for a 0.02mol.dm⁻³ aqueous solution of the redox chromophore, close to maximumcoverage is observed after about 6 h with only a small subsequentincrease in coverage during the following week, see FIGS. 7a and 7 b.Some variability of this process is observed. The electrolyte solutionconsists of LiClO₄ (0.05 mol.dm⁻³) and ferrocene (0.05 mol.dm⁻³) inγ-butyrolactone (BL) (m.p. −45° C., b.p 204° C.). The concentration ofthe LiClO₄ and ferrocene were systematically varied and the results ofthese studies are summarised in FIGS. 8a, 8 b, 9 a and 9 b.

The concentration of added LiClO₄, in the range 0.05 mol.dm⁻³ to 0.20mol.dm⁻³, has no effect on the magnitude of the transmittance change oron the colouring or clearing times (see FIGS. 8a and 8 b). On the otherhand increasing the concentration of added ferrocene, in the range 0.05mol.dm⁻³ to 0.20 mol.dm⁻³, increases significantly the magnitude of thetransmittance change and less significantly the colouring and clearingtimes (see FIGS. 9a and 9 b). The disadvantage of the latter is that theferrocene attacks the epoxy resin seal on the cell and results in devicefailure after about 48 h.

What is claimed is:
 1. A method for the manufacture of an electrochromicsystem using a nanoporous-nanocrystalline film comprising asemiconducting metallic oxide having a redox chromophore adsorbedthereto, wherein the redox chromophore comprises a compound of theformula

wherein X is a charge balancing ion, the method comprising the steps of:(a) providing an electrochromic system comprising a first and a secondelectrode; and (b) disposing said nanoporous-nanocrystalline film onsaid first or second electrode.
 2. A method for the manufacture of anelectrochromic system using a nanoporous-nanocrystalline film comprisinga semiconducting metallic oxide having a redox chromophore adsorbedthereto, wherein the redox chromophore comprises a compound of theformula

wherein X is a charge balancing ion; R₁ is one of the following

and R₂ is one of the following:

the method comprising the steps of: (a) providing an electrochromicsystem comprising a first and a second electrode; and (b) disposing saidnanoporous-nanocrystalline film on said first or second electrode.
 3. Amethod for the manufacture of an electrochromic system using ananoporous-nanocrystalline film comprising a semiconducting metallicoxide having a redox chromophore adsorbed thereto, wherein the redoxchromophore comprises a compound of the formula

wherein X is a charge balancing ion; R₁ is one of the following

and R₂ is

where m is an integer of from 1 to 6 and n is an integer of from 1 to10, the method comprising the steps of: (a) providing an electrochromicsystem comprising a first and a second electrode; and (b) disposing saidnanoporous-nanocrystalline film on said first or second electrode. 4.The method of claim 3, wherein m is 1 or 2 and n is an integer of from 1to
 5. 5. The method of claim 3, wherein R₁ is

wherein n is 2 or
 3. 6. The method of claim 3, wherein m is 1 or 2 and nis an integer of from 1 to 5, and wherein R₁ is

wherein z is 2 or
 3. 7. The method of claim 1, wherein the metallicoxide of the nanoporous-nanocrystalline film is an oxide of a metalselected from titanium, zirconium, hafnium, chromium, molybdenum,tungsten, vanadium, niobium, tantalum, silver, zinc, strontium, iron(Fe²⁺ and Fe³⁺) and nickel and perovskites thereof, preferably TiO₂,WO₃, MoO₃, ZnO or SnO₂.
 8. The method of claim 2, wherein the metallicoxide of the nanoporous-nanocrystalline film is an oxide of a metalselected from titanium, zirconium, hafnium, chromium, molybdenum,tungsten, vanadium, niobium, tantalum, silver, zinc, strontium, iron(Fe²⁺ and Fe³⁺) and nickel and perovskites thereof, preferably TiO₂,WO₃, MoO₃, ZnO or SnO₂.
 9. The method of claim 3, wherein the metallicoxide of the nanoporous-nanocrystalline film is an oxide of a metalselected from titanium, zirconium, hafnium, chromium, molybdenum,tungsten, vanadium, niobium, tantalum, silver, zinc, strontium, iron(Fe²⁺ and Fe³⁺) and nickel and perovskites thereof, preferably TiO₂,WO₃, MoO₃, ZnO or SnO₂.